The present invention relates to fire detectors, and particularly to improvement of electrical or electronic fire detectors.
Electrical or electronic fire detectors are usually arranged so that so-called physical quantity of heat and combustion products or smoke particles produced by fire is detected, said physical quantity being converted by transducer means to a voltage proportional to said physical quantity. When said voltage reaches a predetermined level, an electrical signal is produced to render alarm means operative. In such fire detectors, fire is recognized as occurring only when the output voltage of said transducer means attains said predetermined level within a certain time constant: In other words, only when the output voltage of said transducer means attains said predetermined level within a rated constant time interval.
In known fire detectors of this kind, the output voltage of transducer means which is applied to a constant voltage circuit through a timing circuit has been a superposition of the direct current component and the increasing component. Therefore, conventional fire detectors have such drawback that an erroneous alarm of fire occurs. This occurs, for example, when the relevant fire sensing area is heated by a room heater and the temperature therein rises slowly rather than rapidly to a predetermined temperature which is recognized by the system as fire, or when any room or compartment and the like is filled with cigarette smoke or dust so that its quantity of particles becomes equal to that recognized by the system as indicative of fire.